Page 531 - Practical English Usage 3ed - Michael Swan, Oxford

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When a subordinate clause begins a sentence, it is more often separated by a comma, even if it is short. Compare:
If you are passing, come in and see us. Come in and see us ifyou are passing.
For punctuation in relative clauses. see 495.
leaving words out
Words for repeated ideas can often be left out in the second of two co-ordinate clauses (see 178 for details), but not normally in a subordinate clause. Compare:
She was depressed, and didn't know what to do. (= and she didn't know what to do.)
She was depressed, because she didn't know what to do. (NOT She ItItI:S depFeSSed, hect:tl:tSe didn't knew . ..)
However, after if, when, while, until, once, unless and (al)though, a pronoun subject and the verb be can often be dropped, especially in common fixed expressions like if necessary.
I'll pay for you ifnecessary. (= ... if it is necessary.)
If in doubt, wait and see. (= If you are in doubt ...)
When in Rome, do as the Romans do. Cook slowly until ready. Once in bed, I read for twenty minutes and then turned out the light.
Many conjunctions that express time relations (after, before, since, when, while, whenever, once and until) can often be followed by -ing forms or past participles instead of subjects and full verbs (see 411.6).
I always feel better after talking to you. Some things are neuer forgotten, once learnt.
conjunctions in separate sentences
Normally a conjunction connects two clauses into one sentence. However, sometimes a conjunction and its clause can stand alone. This happens, for example, in answers.
When are you going to get up? - When I'm ready.
Why did you do that?- Because I felt like it.
I'm going out, Mum. -As soon as you've brushed your hair.
Writers and speakers can also separate clauses for emphasis.
This government has got to go. Before it does any more damage.
Afterthoughts may also begin with conjunctions.
OK, I did it. - But I didn't mean to.
For tenses in subordinate clauses. see 580.
For structures in which thar is dropped. see 584.
See also 494-498 (relative pronouns and clauses). 274-278 (indirect speech). and the individual entries on the various conjunctions.
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sentence structure (2): conjunctions 510
page 499